The invention relates to a rotor for an electric machine exited by magnetic poles formed by one or more embedded permanent magnets and to an electric machine exited by such a rotor comprising poles formed by one or more embedded permanent magnets.
Electric machines with permanent magnet (PM) rotors are known in the art. Such PM machines are employed as motors, as generators and as motor-generators. Various ways are known to integrate the permanent magnets in the rotor. PM machines comprising embedded permanent magnets are known to provide, among other things, higher saliency ratios, higher reluctance torque, increased protection against magnet demagnetization and, in case of internal rotor machines, the elimination of an external retaining sleeve around the rotor.
There are generally three types of PM machines rotor topologies known in the art: surface magnet, inset magnet and embedded (buried, interior) magnet rotor topology. In the surface magnet topology, the permanent magnets lie on the rotor lamination surface toward the air gap between rotor and stator in a spoke-like manner orthogonal to the air gap. Air fills the space between the permanent magnets. In the inset magnet topology the permanent magnets also face the air gap between rotor and stator in a similar fashion as for the surface magnet, but iron fills the gap between the magnets. In the third topology, the magnets are not placed in contact with the air gap, as in the previous cases, but they are inserted inside the rotor iron in different configurations. The conventional embedded PM machines utilize narrow bridges to secure the mechanical rotor structure. These bridges can be placed at the air gap or in between the magnets allowing the rotor to rotate at medium or high speeds but still there is usually some flux leakage left.
Slits and air spaces within the rotor lamination are utilized to substantially reduce the flux leakage that occurs in the iron bridges resulting in an improved machine performance.
U.S. Pat. No. 6,888,270 discloses a structurally robust rotor of an electric machine with embedded permanent magnets wherein magnetically non-conductive barrier elements are positioned between the ends of embedded permanent magnets, wherein the rotor is made from powder material.
It is desirable to provide an embedded permanent magnet (PM) rotor with improved performance and possibility to reduce weight compared to typical embedded permanent magnet rotors resulting in an improved machine performance while maintaining the advantages of embedded permanent magnets.
It is also desirable to provide an embedded permanent magnet (PM) electric machine with improved performance compared to a conventional PM electric machine with embedded permanent magnets given the same amount of utilized embedded permanent magnets while maintaining the advantages of embedded permanent magnets.
According to an aspect of the invention, a rotor is proposed for an electric machine excited by magnetic poles formed by one or more embedded permanent magnets, comprising a magnetic body and the one or more embedded permanent magnets associated with the magnetic body defining first magnetic poles and second magnetic poles of alternating magnetic polarity along a rotor direction, wherein for at least one of the one or more embedded permanent magnets a rotor segment is arranged between the one or more embedded permanent magnets and a first surface of the magnetic body. At least one retainer element connects the rotor segment to a portion of the magnetic body.
The rotor segment can be placed in a circumferential direction on a rotor shell for a radial flux machine or on a front face of the rotor for an axial flux machine or a longitudinal extension for a linear machine. Favourably, the retainer elements lock the embedded permanent magnets and the rotor segment directly or indirectly in the magnetic body. The retainer elements may be formed e.g. like wedges which, when seen from a cross section in case of a radial flux machine or from a side view in case of an axial flux machine, protrude in one or more recesses in the magnetic body, or having one or more recesses in which the magnetic body protrudes. For instance, when the retainer element abuts and locks a rotor segment but not the embedded permanent magnet itself, because of being arranged between the rotor segment and the main part of the magnetic body, the embedded permanent magnet will be indirectly locked when the rotor segment is directly locked to the magnetic body by the retainer element. The retainer elements can be advantageously arranged at one or more edges of the embedded permanent magnets. The retainer elements may include magnetically non-conductive areas. In the case of internal rotor design of a radial flux machine such retainer elements have expediently also high mechanical strength for bearing the load caused by centripetal forces. Favourably, the use of magnetically non conducting material can provide a reduction or even elimination of flux leakage.
The invention is applicable to any electric machine topology (e.g. inner and external rotor machines, radial flux machines with internal or external rotors, axial flux machines with internal or external rotors, linear machines, etc), to any kind of embedded rotor topology (e.g. with single embedded permanent magnets or multiple layered embedded permanent magnets, aligned or V-shaped embedded permanent magnet arrangements) and to any shape of permanent magnets (rectangular, breadloaf, etc). The rotor can be manufactured by laminated iron sheets axially stacked along a rotation axis of the rotor (typical for a radial flux machine) or wound around a rotation axis, i.e. stacked in radial direction (typical for an axial flux machine), iron powder material or other ferromagnetic material.
According to a favourable embodiment, a magnetically non-conductive area may be assigned to at least one of the one or more embedded permanent magnets.
Favourably, the retainer element may include the magnetically non-conductive area. In particular, the retainer element constitutes the magnetically non-conductive area. As a result, the shape of the magnetically non-conductive area can be determined by the shape of the retainer element. The retainer element can easily be adapted to required stiffness of the magnetic body of the rotor. For instance, the stiffness can be the same as the stiffness of the magnetic body or can be higher, as required for a desired layout of the rotor, usually considering the forces acting on the components during operation of the electric machine. As the result of the substitution of the rotor material, e.g. iron lamination, with another, magnetically non-conductive material the magnetic rotor loss might also be reduced.
In an advantageous embodiment, the at least one retainer element may lock the rotor segment and/or the one or more embedded permanent magnets by at least one of (i) form locking (ii) frictional locking and/or (iii) the retainer element may be integrally joined with the rotor segment. Favourably, the at least one retainer element can stabilize the position of the rotor segment. A stable arrangement of the embedded permanent magnets and the rotor segment can be achieved.
Additionally or alternatively, the retainer element may engage one or more recesses in the embedded permanent magnet and/or the rotor segment.
According to a favourable embodiment, particularly for a radial flux machine, the rotor segment and/or at least one of the embedded permanent magnets may be fixed in a radial position by the at least one retainer element. The retainer element can be arranged generally inside the magnetic body or, alternatively, at least partially outside the magnetic body.
Favourably, in a radial flux machine the one or more embedded permanent magnets may be arranged alternately in a circumferential direction at the magnetic body and defining first magnetic poles and second magnetic poles of alternating magnetic polarity in the circumferential direction and/or that the rotor segment may be arranged in a substantially radial direction between the one or more embedded permanent magnets and the first surface of the magnetic body. The at least one retainer element can easily be adapted to an actual rotor design of an electric machine. Advantageously, a stable arrangement of embedded permanent magnets and rotor segments on the rotor can be established even at high rotational speeds of the rotor as well as at high forces generated by high electric currents in the stator coils. Favourably, the first surface of the magnetic body is provided for facing a stator. In case of an internal rotor the first surface is the outer surface of the rotor. In case of an external rotor, the first surface is the inner surface of the rotor. Particularly, the first surface is the shell surface at the outside of the rotor (in case of the rotor is provided for being surrounded by the stator) or the shell surface at the inside of the rotor (in case the rotor is provided for surrounding the stator).
Generally, for all types of electrical machines and rotors, the at least one retainer element can be arranged on both the external sides of each embedded permanent magnet and/or each magnet pole. The purpose of the retainer element is to lock the rotor segment and the permanent magnet and keep them fixed in a position as well as, when comprising magnetically non-conductive areas, to reduce or eliminate flux leakage. In the case of external rotor design of a radial flux machine, where a stator is surrounded by the rotor, the rotor segments and embedded permanent magnets are arranged at the inside of the rotor, provided for facing the stator. As centrifugal forces are directed to the outside of the rotor, the centrifugal forces act on the embedded permanent magnets and/or rotor segments in a way to stabilize their radial positions. As a result, the purpose of the retainer element in such an external rotor arrangement is mainly to reduce or eliminate flux leakage.
According to a favourable embodiment, particularly for a rotor for a radial flux machine, one or more embedded permanent magnets may be stacked in radial direction of the magnetic body with a radial distance to each other. This is particularly of advantage for rotor for a radial flux machine with multilayered embedded permanent magnets. In one arrangement with multilayered embedded permanent magnets, an embedded permanent magnet is sandwiched between two rotor segments in radial direction wherein two or more embedded permanent magnets can be provided each sandwiched between rotor segments. Retainer elements can be arranged at the sides of the embedded permanent magnets and/or the rotor segments, each locking the embedded permanent magnet and/or the rotor segments in its/their position in the magnetic body. A retainer element can be arranged between two embedded permanent magnets, and/or at each of the opposite outer ends of the embedded permanent magnets, depending on the desired design of the rotor. In another arrangement, retainer elements can be arranged only at the outer edges of the embedded permanent magnets in the circumferential direction.
According to a favourable embodiment, particularly for a rotor for an axial flux machine, the rotor segment and/or at least one of the embedded permanent magnets may be fixed in an axial position by the at least one retainer element. This is advantageous for a rotor for an axial flux machine. The rotor segment is securely fastened independent on an inclination of the electric machine.
According to a favourable embodiment, particularly for a rotor for an axial flux machine, the one or more embedded permanent magnets may be arranged alternately in at least one front face of the magnetic body and defining first magnetic poles and second magnetic poles of alternating magnetic polarity on the front face of the magnetic body. The rotor segments may be arranged in a substantially axial direction between the one or more embedded permanent magnets and the first surface of the magnetic body. The rotor segment is securely fastened in the axial direction independent of the orientation of the electric machine during use. A proper shape of the retainer element together with high mechanical strength could secure the embedded permanent magnet position also in the radial direction, even at a high rotational speed of the rotor.
In another advantageous embodiment, the at least one retainer element may have an outer edge arranged flush with the first surface of the magnetic body. The surface of the rotor can easily be trimmed during manufacturing.
In an alternative embodiment, the retainer element can be arranged non-flush with said first surface. In such an arrangement the retainer element sticks out from the magnetic body towards the air gap. This solution can also improve the cooling in the air gap as the protruding parts can act like vanes of a fan.
Further, as a result of the outer edge of the retainer element arranged flush with the first surface of the magnetic body, magnetic losses caused by magnetic flux quenched in a portion of the magnetic body between the magnetically non-conductive area and the first surface of the magnetic body can be reduced, particularly if ceramic or other non-electrically conductive material is utilized instead of carbon fibre. Due to the overlap region the embedded permanent magnet is retained directly or indirectly in its radial position by the rotor segment in case of a rotor for a radial flux machine (even at high rotational speeds). The retainer element not only retains the embedded permanent magnet (and the rotor segment) in its position in the magnetic body but can also stabilize the embedded permanent magnet in order to avoid vibrations during rotation of the rotor as well as against forces acting on the embedded permanent magnet generated by electrical currents flowing in the stator windings. Favourably, it is possible to reduce deterioration of the rotor characteristics. For instance, in case tolerances are too high among the three different types of elements concerned, i.e. the embedded permanent magnets, the retainer elements and the lamination, unwanted gaps that might occur during manufacture of the rotor can be filled with epoxy added to the rotor body.
Expediently, the retainer element(s) can be arranged at an outer edge of one or more of the embedded permanent magnets. Thus, the rotor segment may substantially cover an edge of the embedded permanent magnet. A reduction of the magnetic body's effective magnetic area and/or volume due to the introduction of the retainer elements should account for the tolerances of the different elements (components) in the design of the rotor.
It is of advantage if the embedded permanent magnet in the magnetic body can be provided with such a retainer element at each end of its extension in the circumferential direction of the rotor. The arrangement of the retainer element with respect to the embedded permanent magnet allows trimming the first surface of the magnetic body by removing material from the shell surface in order to provide a flush relationship of the retainer element with the first surface. Preferably, the first surface is provided for facing a stator. Expediently, each embedded permanent magnet arranged in the magnetic body of the rotor is provided with at least one rotor segment. It is expedient if all embedded permanent magnets in the rotor are provided with at least one retainer element, particularly on both free ends, e.g. both circumferential ends, of each embedded permanent magnet. The shape of the retainer element can be optimized in relation to desired magnetic parameters and mechanical strength of the rotor and may be adapted to the number, size and shape of the embedded permanent magnets, the radial position of the embedded permanent magnets and other design parameters determining the characteristics of the rotor.
An internally arranged retainer element can be provided if the at least one retainer element is arranged between two adjacent embedded permanent magnets. At the outer edges, it is possible to arrange further retainer elements or even to arrange air pockets. In the latter case, the weight of the rotor can be further reduced.
In an expedient embodiment, the at least one retainer element can be arranged in an axial slot in the magnetic body. Generally, retainer elements in the rotor can be made with identical cross sections. However, depending on the rotor design, retainer elements of different cross sections may also be used. However, retainer elements arranged at opposing ends of a magnet pole are usually of mirror-symmetric shape.
According to a favourable embodiment, the at least one retainer element may comprise at least one of carbon fibre, carbon fibre composites, glass fibre, glass fibre composites, polymer fibre, such as e.g. aramid fibre, polymer fibre composite, ceramics, plastics with mechanical strength similar or superior to the lamination. Particularly, the material may be a magnetically non-conductive material. The retainer element can be manufactured separately from the magnetic body or the embedded permanent magnet. As a result, the retainer element can be shaped according to the actual shape and/or arrangement of the embedded permanent magnet in the magnetic body of the rotor. Carbon fibre composite for instance is robust and has a low specific weight. Expediently, the retainer element made of carbon fibre composite can be configured to have reduced electrical conductivity, e.g. by depositing a material such as a varnish-polymer resin on the surface the retainer element.
According to another favourable embodiment, the at least one retainer element may abut a section of all outer contour of one or more embedded permanent magnets and/or may engage a recess in the embedded permanent magnet and/or in the magnetic body and/or in the rotor segment. Additionally or alternatively, the embedded permanent magnet and/or the magnetic body and/or the rotor segment may engage a recess in the retainer element. By such an interdigital or toothed arrangement of the retainer element in relation to the embedded permanent magnet and/or the magnetic body and/or the rotor segment, each said component can be directly or indirectly secured reliably in its radial position even at very high rotational speeds or high currents in the stator windings.
According to another favourable embodiment, the magnetic body can be made of stacked laminates. In this embodiment, openings for the embedded permanent magnets and retainer elements can easily be manufactured by punching from sheet material. This manufacturing step is particularly useful for big volume series production of such rotors. An expedient manufacturing process includes the steps of (i) punching the rotor lamination so that the magnetic body and the rotor segment are at the beginning attached by small iron bridges; (ii) placing the embedded permanent magnets and retainer elements in respective slots of the magnetic body, (iii) removing the iron bridges, e.g. by grinding.
According to another favourable embodiment, the magnetic body can be made of iron powder. Openings for the embedded permanent magnet can be provided in the sinter form. The magnetic body can be manufactured by sintering the iron powder. It is expedient if at least one retainer element can be made from powder and co-sintered with the magnetic body. In such a case, the magnetic body and the retainer element can be manufactured in one step. As a result, manufacturing tolerances between openings for the retainer element and the retainer elements can be reduced. In an expedient embodiment, the rotor segment and the retainer elements associated with the rotor segment can be co-sintered and arranged in the rotor magnetic body, which can consist of or comprise stacked laminates or be sintered from iron powder material.
According to another favourable embodiment, the retainer element can be supported by one or more bandages retaining the rotor segment at its position at the magnetic body. This is advantageous in cases when not very high load is exerted to the rotor and in cases the retainer element is designed to be comparably small.
Another aspect of the invention relates to an electric machine with a stator and being excited by a rotor with magnetic poles formed by one or more embedded permanent magnets, according to any one of the features described above.
Favourably, the rotor of the electric machine can provide anyone of the advantages described above.
The electric machine may have the rotor being configured for a radial flux machine, an axial flux machine or a linear machine.
Additionally or alternatively the rotor for a radial flux machine may be configured as an external rotor surrounding the stator or the rotor may be configured as an internal rotor surrounded by the stator, or the rotor for an axial flux machine or for a linear machine may be configured as external rotors with a stator arranged between two external rotors or being configured as an internal rotor enclosed by two stators.
The retainer element can be easily arranged and shaped to its desired application in the electric machine. Such a design for an internal or external rotor refers to radial flux machines. The machine can also be designed as axial flux or linear machine. In case of multiple layers of embedded permanent magnets there can be two layers of embedded permanent magnets above and another in a layer below.
The electric machine is advantageous for various applications where low losses and high torque densities are required, expediently as a generator in a vehicle or as motor for driving a drive train in a vehicle.